1. Field of the Invention
The invention relates to a piston pump which is particularly provided for a hydraulic vehicle brake system that has a traction control system.
2. Description of the Prior Art
A piston pump of this kind has been disclosed by DE 197 32 817 A1. The known piston pump has a piston which is contained in an axially mobile fashion over a part of its length in a sleeve. Together with the sleeve, the piston can be inserted into a pump bore of a pump housing, where the piston, in a section protruding from the sleeve, is guided in an axially mobile fashion in the pump bore in the pump housing. A tubular filter, which encompasses the piston in its section protruding from the sleeve, is placed onto the sleeve in such a way that it extends the sleeve. The filter constitutes a captive retainer, which engages behind an undercut, for example an annular step of the piston, and thus secures the piston in the sleeve. The sleeve, the piston, and the filter securing the piston in the sleeve comprise a preassembled unit, which is assembled outside the pump housing and is then inserted into the pump bore in the pump housing. The filter constituting the captive retainer is intended to secure the piston in the sleeve until the preassembled unit is inserted into the pump bore in the pump housing. When the preassembled unit is inserted into the pump housing, the piston is secured in the sleeve by means of a cam that drives it so that it is no longer necessary for the filter to retain the piston in captive fashion.
A sealing ring is inserted in order to seal the piston in the pump housing and a guide ring is inserted into the pump bore in the imaginary extension of the filter in order to axially guide the piston in the pump housing. In order to compensate for length tolerances, in the known piston pump, a comparatively large amount of play must be provided in the axial direction between the filter and the sealing ring; the sealing ring has a large amount of axial play. This has the disadvantage that the sealing ring can move toward the filter during an intake stroke of the piston pump, which reduces an intake volume and thereby reduces an efficiency of the piston pump. In addition, due to an axial movement during the stroke motion of the piston, the sealing ring can convey a leakage flow from the piston pump into a cam chamber and can aspirate air into the piston pump in the reverse direction, which likewise impairs pump performance.
In the piston pump according to the invention, the filter is attached to the sleeve in an axially mobile fashion. This results in the fact that the filter assumes its final axial position in relation to the sleeve only when the sleeve is inserted together with the piston and filter into the pump bore in the pump housing. The invention achieves a tolerance compensation in the axial direction between the sleeve and the filter attached to it that permits an axial play for a sealing ring, which seals the piston in the pump housing, to be embodied within strict tolerances. This has the advantage that an undesirable axial mobility of the sealing ring can be reduced and can even be eliminated, provided that the sealing ring permits this. This prevents leakage fluid from being conveyed by axial movement of the sealing ring due to a stroke motion of the piston of the piston pump. In addition, a wear of the sealing ring is reduced and the sealing ring is prevented from reducing the intake volume of the piston pump according to the invention by moving axially during an intake stroke of its piston.
The piston pump according to the invention is particularly provided as a pump in a brake system of a vehicle and is used to control the pressure in wheel brake cylinders. Depending on the type of brake system, these brake systems are referred to by the abbreviations ABS, TCS, ESP, or EHB. In the brake system, the pump is used, for example, to return brake fluid from one or more of wheel brake cylinders to a master cylinder (ABS) and/or for supplying brake fluid from a reservoir into one or more of wheel brake cylinders (TCS, ESP, or EHB). The pump is required, for example, in a brake system with a wheel slip regulation (ABS or TCS) and/or in a brake system used as a steering aid (ESP) and/or in an electrohydraulic brake system (EHB). Wheel slip regulation (ABS or TCS) can, for example, prevent the wheels of the vehicle from locking when there is powerful pressure on the brake pedal during a braking maneuver (ABS) and/or can prevent the driven wheels of a vehicle from spinning when there is powerful pressure on the accelerator pedal. In a brake system used as a steering aid (ESP), a brake pressure is built up in one or more wheel brake cylinders, independent of an actuation of the brake pedal or accelerator pedal, in order, for example, to prevent the vehicle from swerving out from the path desired by the driver. The pump can also be used in an electrohydraulic brake system (EHB) in which the pump supplies brake fluid to the wheel brake cylinder(s) when an electric brake pedal sensor detects an actuation of the brake pedal or in which the pump is used to fill a reservoir of the brake system.